Dental materials containing polymerizable compounds such as a (meth)acrylic ester compound and the like have been widely used in the field of dental treatment from the advantages of excellent operability, aesthetic property, strength and the like. In particular, visible light polymerization type dental compositions have been used in many cases because they are capable of using visible light which is safe to living body and they have the aforementioned advantages (Patent Document 1).
As fillers contained in the dental compositions for composite resins, hard resins, artificial teeth or the like, powdery inorganic components such as silica, glass and the like are used for the purposes of imparting the mechanical strength to the compositions and improving various properties of a resin matrix from the past.
In the dental treatment, the condition of the treatment is often confirmed by X-ray photography, so that as a powdery inorganic component contained in the dental compositions, a powder (a pulverized glass powder) obtained by pulverizing a glass containing a heavy metal element having X-ray contrast property, such as barium, zirconium, strontium or the like, has been heretofore used such that X-ray photography should be feasible in the dental treatment. The pulverized glass powder is generally prepared by, for example, pulverizing such a glass as mentioned above. In the conventional technique for glass pulverization, however, it was difficult to finely pulverize a glass, and a glass powder having a particle diameter of around 10 μm to several tens of μm (20 to 30 μm) has been usually employed. If a dental composition containing a glass powder having such a large particle diameter was used, it was very difficult to form a machined surface having polish clinically similar to that of natural teeth.
In order to solve the aforementioned problems associated with incorporation of such a glass powder, composite resins mainly using a finely pulverized glass powder having an average particle diameter of not more than 2 μm have been recently developed. For example, an invention using a pulverized glass powder having an average particle diameter of from 0.1 to 5 μm and/or an inorganic compound having an average particle diameter of from 0.01 to 0.04 μm, such as silica fine particles or the like, is disclosed (Patent Document 2).
The composite resin using the finely pulverized glass powder is improved in the surface polish that has been considered as a disadvantage of a composite resin using a conventional pulverized glass powder having a relatively larger particle diameter, but the dental composition containing such a finely pulverized glass powder still has many disadvantages to be improved, with regard to the balance of properties such as transparency, photocurability, X-ray contrast property and the like. Namely, for example, in case of a dental composition wherein a finely pulverized filler having an average particle diameter of not more than 2 μm is homogeneously contained, the boundary area between the filler and the resin matrix (cured resin product) is markedly increased, and in order to ensure its transparency, refractive indices of the filler and the resin matrix need to be approximated to each other. However, if the content of the heavy metal element in the filler is increased to ensure high X-ray contrast property, the refractive index of the filler becomes high. The refractive index of the resin matrix such as epoxy methacrylate (hereinafter referred to as Bis-GMA for short) to be derived from 2,2-bis(4-hydoroxyphenyl)propane (usually called bisphenol A) that has been heretofore applied to dental use is at most about 1.55, and by approximating the refractive index of the filler to the aforementioned value, the transparency of the dental composition has been ensured. However, fillers having high X-ray contrast property and suitable polymerizable compounds imparting a cured resin product having high refractive index have been in demand, some of which have been proposed (Patent Documents 3 to 4).
As described above, with regard to the transparency of teeth, extremely high transparency in the appearance is often required for the composite resin or the hard resin used for the front tooth broken edge or the like. In such a part, the light transmittance at the wavelength of 480 nm that is a wavelength of a light from a dental irradiator needs to be not less than 5% in many cases. On the other hand, in the dentin portion or the like, the transparency of the dental resin or the hard resin may be relatively low because such a part is free from such problems in the appearance, and it is possible to use even a dental resin or a hard resin having a transparency of not more than 1%. In order to control color tone of the teeth, however, a pigment such as titanium oxide, bengala or the like is combined, so that even the dentin is desired to have high transparency to enhance the degree of freedom of coloring. In case of a dental material of a photopolymerization type, there is an advantage that higher transparency brings about larger curing depth and higher conversion, and as a result, the mechanical properties are enhanced.
The light transmittance of a dental composition containing no pigment is preferably not less than 0.05%, more preferably not less than 1%, and in the practical use, the light transmittance is particularly preferably not less than 5%. The X-ray contrast property value of such a material commercially available at present is at most about 200 to 300% based on aluminum, so that it is feasible to distinguish such a material from the tooth enamel (X-ray contrast property value of about 180% based on aluminum) by X-rays in the usual filling treatment. In case of filling in a small amount or thin filling, however, it becomes difficult to clearly distinguish such a material from the tooth enamel.
Furthermore, as a method for decreasing polymerization shrinkage that is a clinically serious problem in the dental material, use of a dental composition mixed with a composite filler obtained by pulverizing a cured product obtained by curing a mixture of an inorganic filler and a polymerizable composition in advance is known. Although the dental composition can achieve a decrease in the polymerization shrinkage, refractive indices of the inorganic filler, a resin matrix of the composite filler and a resin matrix of the dental composition need to be finely suited to each other in order to ensure the transparency in case of the aforementioned composite filler.
Under these circumstances, in recent years, such dental materials and dental compositions having high X-ray contrast property and transparency, and at the same time, small polymerization shrinkage, and satisfying various properties (for example, mechanical strength, water absorption and the like) required for dental materials have been in demand. Also, new polymerizable compounds capable of realizing these materials have been demanded.
Patent Document 1: JP1973-49875A
Patent Document 2: JP1993-194135A
Patent Document 3: JP1996-157320A
Patent Document 4: JP1996-208417A
On the other hand, an inorganic glass has been used in a broad range of fields as a transparent optical material because it is excellent in the transparency and various properties such as small optical anisotropy or the like. However, such an inorganic glass has drawbacks in that it is heavy and easily broken, and it has bad productivity in processing for the production of optical parts or the like. As a result, a transparent organic polymer material (optical resin) has been under development with eagerness as a material for replacing the inorganic glass. Recently, high functionality and high quality in optical resins have been promoted. Optical parts obtained by molding and processing these optical resins have further come into wide use, for example, in lens fields such as a spectacle lens for vision correction, a pickup lens in optical information recording devices such as CD, DVD and the like, a plastic lens for cameras such as a digital camera and the like, and a Fresnel lens for use in a liquid crystal projector or a projector television.
One of the most important and fundamental properties as an optical resin is transparency. Up to now, as an optical resin having excellent transparency, there have been known resins, for example, polymethyl methacrylate (PMMA), polycarbonate (BPA-PC), polystyrene (PS), methyl methacrylate-styrene copolymer (MS), styrene-acrylonitrile copolymer (SAN), poly (4-methyl-1-pentene) (TPX), polycycloolefin (COP), diethylene glycol bisallylcarbonate polymer (DAC); polythiourethane (PTU), and the like.
Among these optical resins, polymethyl methacrylate (PMMA) is widely used as one of representative optical resins as it is superior in its transparency and has characteristics such that optical anisotropy is low (low double refractive index), and moldability and weather resistance are good, and the like. However, there are drawbacks in that its refractive index (nd) is low, i.e., 1.49, the water absorption coefficient is high, and the like.
In the same manner, polycarbonate (BPA-PC), one of the representative optical resins, can be obtained by the polycondensation reaction of 2,2-bis(4-hydroxyphenyl)propane (hereinafter referred to as bisphenol A what is commonly called) and a carbonate compound (for example, carbonyl chloride, diphenylcarbonate or the like), having characteristics such that the transparency, heat resistance and impact resistance are excellent, and the refractive index (nd=1.59) is relatively high. As a result, it is widely used for optical purposes including an optical disk plate for information recording. However, it has drawbacks in that the chromatic aberration (dispersion of refractive index) and double refractive index are relatively high, and melting viscosity is high, thus lowering moldability a little. Needless to say, improvement of new performance and characteristics is being made to overcome these drawbacks.
Diethylene glycol bisallylcarbonate polymer (DAC) is a thermosetting resin of a crosslinked high molecular structure to be obtained by casting radical polymerization of a monomer, i.e., diethylene glycol bisallylcarbonate. It has characteristics such that the transparency and heat resistance are excellent, and the chromatic aberration is low. Due to such characteristics, it is used the most for a general-purpose plastic spectacle lens for vision correction. However, there are drawbacks in that its refractive index is low (nd=1.50) and its impact resistance is a little inferior.
Polythiourethane (PTU) is a thermosetting resin of a crosslinked polymer structure to be obtained by the reaction between a diisocyanate compound and a polythiol compound. It is an extremely excellent optical resin having characteristics such that the transparency and impact resistance are excellent, the refractive index is high (nd≧1.6), and the chromatic aberration is relatively low. At present, it is used the most for the purpose of a high-quality plastic spectacle lens for vision correction in which the thickness is thin and the weight is light. However, there is room for further improvement only from the viewpoint of the productivity requiring long time (1 to 3 days) for thermal polymerization molding in the production process of spectacle lenses.
In order to solve these problems and to produce optical parts such as optical lenses and the like with high productivity, there has been proposed or suggested a method of polymerizing and molding a compound having radical polymerization ability (hereinafter referred to as a polymerizable compound) for obtaining an aiming molded product within a short period of time in the presence of a compound (photopolymerization initiator) for initiating radical polymerization by an irradiation with a light such as ultraviolet rays and the like (Patent Documents 5 to 8 and the like).
As a representative example of the polymerizable compound used for the photopolymerization, a (meth)acrylic ester compound is used. However, there has been proposed a (meth)acrylic ester compound, a (meth)acrylic (thio)ester compound or the like having a specific structure containing a bromine atom or a sulfur atom in order to obtain much higher refractive index and Abbe number. According to these methods, such a polymerizable compound can be polymerized within a short period of time. However, considering the balance of general properties such as transparency, optical properties (for example, refractive index, Abbe number or the like), thermal properties (for example, heat distortion temperature or the like), mechanical properties (for example, impact resistance, flexural strength or the like) and the like, it is difficult to mention that the thus-obtained cured product (resin) is sufficiently satisfying as an optical part.
Conventionally known optical resins have excellent characteristics as described above, but, according to the present state, each of them has also drawbacks to overcome. Under these circumstances, it is required to develop optical resins such that they can be polymerized and molded within a short period of time by photopolymerization, in which the transparency and optical properties (high refractive index and Abbe number) of the thus-obtained cured product or optical part are excellent, and thermal properties, mechanical properties and the like are good.
Patent Document 5: JP1992-180911A
Patent Document 6: JP1988-248811A
Patent Document 7: JP1988-207632A
Patent Document 8: JP1986-194401A
An object of the present invention is to solve the above problems in a dental material and a dental composition. That is, the present invention is to provide a dental material and a dental composition which satisfy the requirements for dental materials and dental compositions, which have excellent curability and well-balanced excellent transparency and excellent X-ray contrast property though the transparency and the X-ray contrast property are hardly compatible with each other, and have small polymerization shrinkage, and a polymerizable compound capable of realizing such a dental material or a dental composition.
Furthermore, another object of the present invention is to provide an optical resin which can solve the above problems associated with optical resins for use in optical parts, more specifically, a polymerizable composition which can be polymerized and molded within a short period of time by photopolymerization and imparts a cured resin product having excellent transparency, optical properties (refractive index and Abbe number), heat resistance, mechanical properties, weather resistance and the like, and an optical part obtained by polymerizing the above polymerizable composition.